The present invention relates generally to particulate detection systems for determining particulate levels in fluid samples. More specifically, the present invention relates to such systems that utilize filter assemblies, wherein a membrane filter is retained within a filter capsule having an inlet and an outlet to permit fluid to pass through the filter and deposit suspended particulates thereon for further analysis.
A variety of particulate detection systems are currently used in laboratories and in field operations to determine particulate concentrations in various fluid testing applications. For example, an important application for a particulate detection system is the testing of aircraft fuel supplies for unacceptable levels of particulate contamination. Many of the detection systems currently in use are based upon one of several conventional techniques utilizing a membrane filter on which particulates are deposited. The membrane filter is analyzed to determine the amount of particulates on the filter, thereby indicating the concentration of particulates in the fluid sample. The most common methods currently used to analyze the membrane filter are Gravimetric Assessment, Colormetric Assessment, or Visual Assessment.
The Gravimetric Assessment method of analyzing the membrane filter involves actual weighing of the particulate products retained on the membrane filter after the fluid has been passed therethrough. The Colormetric Assessment method involves evaluating the particulate laden membrane filter on the basis of coloration, hue, chroma, and intensity. The Visual Assessment method relies upon a skilled operator to correctly identify the quality and quantity of particulates deposited on the membrane filter.
As previously noted, particulate detection systems are used in field testing of aircraft fuel supplies. One such fuel testing system is the Accumetric fuel contamination detector, manufactured by TMI of Fort Wayne, Ind., which utilizes the principle of differential light transmission through membrane filters to measure particulate contamination levels. More specifically, two membrane filters are used in series, wherein the first filter traps particulates and acquires fuel coloration while the second filter acquires only fuel coloration. The Accumetric fuel contamination detector processes differential readings from the filters, automatically compensates for fuel coloration, and indicates a particulate contamination level.
In each of the aforementioned methods of analyzing a membrane filter to determine the particulate count, the accuracy of the measurement is affected by uneven particulate distribution on the sample filter. One primary cause of such uneven particulate distribution is the design limitations of the sampling equipment used to house the membrane filter. A typical monitor capsule used in fuel sampling includes an inlet half having a centrally located inlet port, an outlet half having a centrally located outlet port, and a membrane filter retained between the halves, through which liquid fuel is passed. According to this conventional design, a stream of fluid enters the inlet port and impacts directly against a central portion of the membrane filter face. Consequently, the fluid pressure across the filter face varies greatly, resulting in a "halo" of concentrated particulates at the outer periphery and a "hole" of relatively clean filter area in the center of the filter face.
The present invention is directed to overcoming the aforementioned problems associated with prior art filter assemblies for particulate detection systems, wherein it is desired to provide an improved membrane filter capsule that is capable of providing substantially uniform fluid pressure distribution across the face of the membrane filter, thereby resulting in even particulate distribution and improved accuracy of the associated particulate detection system.